Treatment of mouse NIH 3T3 with interferon (IFN) inhibited their transformation by transfection with Harvey murine sarcoma virus DNA (Ha-MusSV DNA) or the human EJ/T24 c-Ha-ras oncogene. Transformation was significantly inhibited even when IFN was added 8 days after transfection. Furthermore, treatment with IFN of RS 485, an NIH 3T3 line already transformed by a human c-Ha-ras gene activated by a Ha-MuSV promoter resulted in a phenotypic reversion in the biological properties of these cells. The revertant RS 485 resembled untransformed NIH 3T3 with respect to morphology, growth in soft agar, and saturation density, and in preliminary experiments had reduced tumorigenicity in nude mice and produced less of an oncogene-coded protein (p21) associated with transformation, when compared to RS-485. The biological and biochemical aspects of human oncogene expression will be further studied in these revertants including their levels of c-Ha-ras specific DNA, mRNA, and p21. In addition I shall study reversion following IFN treatment in RS 504, a cell line transformed by the EJ/T24 oncogene that has a nonviral promoter. Both RS 485 and RS 504 will be cotransfected before IFN treatment with the neo and SLAd, genes that are usually expressed in mouse cells in order to test whether IFN-induced inhibition of oncogene expression is selective. I shall also investigate whether the observed inhibition of oncogene expression is related to changes in the response of cells to IFN or in the well-characterized mechanisms of action of IFNs that involve a specific protein kinase and the 2'5' oligoadenylate system. In order to study these I shall assay the biological responses of transformed revertant cells to IFN and test the basal and induced levels of the various IFN-associated enzymes in these cells. I shall also study reversion in NIH 3T3 cells deficient in protein kinase or the endonuclease associated with the 2'5'A system. After treatment with IFN some previously reverted RS 485 "retransform" to a transformed state; however, the great majority of revertant RS 485 cells remain in a persistently revertant state. Oncogene expression and IFN-induced biochemical changes will also be studied in retransformants and persistent revertants. I hope to demonstrate that IFNs modulate the expression of human oncogenes, thus establishing a basis for their use in treatment of human cancers. (X)